Coil Tests

[ddomino]

RE: Coil test data clarification

Couple more questions:

Can coil inductive reactance be directly measured, or failing that, can it be computed from measurable data? What data points would need to be measured? Would we need to know the output frequency of the alternator to perform the computation?

To the extent that inductive reactance is a kind of resistance, is it a contributor to heat build-up in the coil?

Thanks again,

dd

Later edit: So to close out my understanding, the monotonic decrease in amps with increasing RPM is a function of the increasing inductive reactance within the coil due to increasing RPM?.

Would the "computed resistance" (making up a term) be a value computed by dividing the voltage by the measured amps? For example, for the Black Standard coil V/I for 1500 rpm yields 13.8V/1.60a= 8.63 ohms. At 2000 rpm the computed resistance is 9.72 ohms. Would inductive reactance in ohms be equal to the total computed ohms minus the measured static value?

Thanks again for listening to this ramble...I'm retired now, you see (g).

[joe_db]

The alternator - unless badly defective - should have no output frequency. It puts out DC with a bit of ripple from the 3-phase bridge rectifiers that is smoothed out by the battery.
The second part can be fairly complex to model if you want to be a real EE geek and get all the variables correct.
To start with as a pure DC system, once the points start opening and closing (or their electronic equivalent), current is only flowing part of the time instead of all the time. Dwell is usually the way this is expressed for ignition systems and the more dwell the more on time. To add another variable, rapidly switching current into an inductor is no longer just a DC problem. If it were, RPM would have no influence because if the points are on half the time they will be on half the time at 100 RPM or 4000 RPM and the average current would be be same.
The AC part comes in with back EMF and inductance/reactance both as variables that can change with frequency. I actually don't have the energy to dig out all my old EE books and get every detail, but these issues tend to reduce current flow with higher RPMs. Back EMF is the nature of a coil, when it is energized the building magnetic field produces a current that opposes the incoming current. It takes a finite amount of time for the current to build to maximum. See http://hyperphysics.phy-astr.gsu.edu...ic/indtra.html for some detail. The higher the RPM the more this is a factor because the coil spends less time at its maximum possible current. I honestly forget how much of RPM/Current curve is determined by back-EMF factors and how much is modeled on frequency vs. reactance.
As a practical matter: the coil's worst case is being on with the engine not running. The next worst case is running slowly. Using EI makes it worse because the ratio of on to off is more on than with points. In cases where coil current is high enough to shorten the life of the coil we use ballast resistors to reduce it. Back in the day many engines used ballast resistors and had a bypass connection on the solenoid to supply full voltage when starting. I think all the "new" type starters have this connection if anyone wants to recreate this system. Also keep in mind different use scenarios. I have run my engine over 48 hours nonstop adding gas from cans as we went. Some people motor 10 minutes off a mooring to daysail and don't use 5 gallons of gas in a year.

Couple more questions:

Can coil inductive reactance be directly measured, or failing that, can it be computed from measurable data? What data points would need to be measured? Would we need to know the output frequency of the alternator to perform the computation?

To the extent that inductive reactance is a kind of resistance, is it a contributor to heat build-up in the coil?

Thanks again,

dd

[ddomino]

Thanks Joe. The reason I asked about frequency is that the online sources I checked on inductive reactance all had frequency as a variable in their computations. Not all of it was clear to me in any case, but that was the source of my question. I know that by the time the energy gets to the boat it has been rectified to DC, but I wondered whether the computation would require "pre-rectified" frequency. In any case I think I have it "clear enough for government work". I posted a couple of edits to my earlier queries while you were posting yours, so thanks.

dd

[hanleyclifford]

Hanley,

Your own instrument panel shows that the current through your coil is similar to my test results. Why would you think that the coil would fail? The coils I tested all came from MMI and Don has stated that he has never heard of one of his coils failing.

Tim, Thanks for the testing. I've never tried to examine this matter on the level you and Joe, and others, do but maybe I should . I notice that you measure amperage between coil- and the igniter whereas I have mine connected in front of coil+. Maybe I'm missing something here. Since I have extra gauges, next time at the boat I will connect one as you have and report back any new data. Thanks again for your detailed approach to this subject.

[Tim]

Hanley,

The circuit is in series. The current should be the same everywhere in the circuit.

[hanleyclifford]

Hanley,

The circuit is in series. The current should be the same everywhere in the circuit.

That is the hypothesis I want to test. I wonder if some current is being dissipated in the coil (via heat?) which might explain why both you and Neil might be right.

[joe_db]

The frequency in those formulas would correspond to the RPM of the engine more or less.
You can run through the calculations here to get some ideas about current rise in coils:

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/indtra.html

Thanks Joe. The reason I asked about frequency is that the online sources I checked on inductive reactance all had frequency as a variable in their computations. Not all of it was clear to me in any case, but that was the source of my question. I know that by the time the energy gets to the boat it has been rectified to DC, but I wondered whether the computation would require "pre-rectified" frequency. In any case I think I have it "clear enough for government work". I posted a couple of edits to my earlier queries while you were posting yours, so thanks.

dd

[ndutton]

Watching to see where this is headed and what, if any, conclusions result from the testing. Is there a trend afoot to abandon the Rule of 2011? Plan on going back to 3Ω coils with EI?

No worries here, I have a reliable installation proven over several prolonged trips under power and a spare engine that is equally reliable so I'm good.

[Tim]

Neil,

A coil with 3 ohms or less of internal resistance will have a high risk of failure and the coil test proves it. In the test, the blue Standard UC-15 coil had an internal resistance of 3.3 ohms and prior to starting the engine there was 3.75 amps with 12.5 volts on the coil +. A coil with only 3 ohms of internal resistance would exceed the 4 amp limit for the EI module.
The coil test results show:
1) The highest current occurs with the ignition switch on and the engine off.
2) Due to the nature of coil operation and magnetic fields, the current in the coil is lower when the engine is running and the current decreases as RPM increases.
3) The highest risk of coil damage is when the ignition switch is on and the engine is off.

A coil is a long thin wire coated in a very thin layer of insulating material and is usually wound around an iron core. Exceeding the current limit, even for brief periods, can damage the insulating material. Once the insulating material is damaged, the coil will fail - sometimes quickly and sometimes more gradually.

[ndutton]

Thanks for explaining coils and the effect of internal heat to me.

[roadnsky]

????

Neil,
A coil with 3 ohms or less of internal resistance will have a high risk of failure and the coil test proves it. In the test, the blue Standard UC-15 coil had an internal resistance of 3.3 ohms and prior to starting the engine there was 3.75 amps with 12.5 volts on the coil +. A coil with only 3 ohms of internal resistance would exceed the 4 amp limit for the EI module.
The coil test results show:
1) The highest current occurs with the ignition switch on and the engine off.
2) Due to the nature of coil operation and magnetic fields, the current in the coil is lower when the engine is running and the current decreases as RPM increases.
3) The highest risk of coil damage is when the ignition switch is on and the engine is off.

I thought we learned all of this back in 2011?
AND wasn't our professor... Neil?!

[Tim]

Don sent me three coils and asked me to test them. Don was curious as to why his coils don't seem to need ballast resistors and have a very low failure rate. What we were looking for was the voltage and current in the primary circuit under various operating conditions. I tested the three coils plus the one I already had. Don asked me to post the results on the forum. The results of the test do confirm what was already known - a quality coil with 3.25 ohms of internal resistance should not need a ballast resistor, a coil with less than 3.25 ohms of internal resistance will need a ballast resistor to reduce risk of failure.

Don was also very interested in the dynamic nature of coils and we had a very detailed exchange about current, impedance, resistance, inductive reactance and the effects of changing frequency in a coil.

[ndutton]

Tim, please consider making a blanket statement that a quality 3.25Ω coil does not need a ballast resistor is hugely dependent on coil input voltage. One particularly vocal critic of our EI/coil work prefers his alternator voltage to be 14.8 volts (his measurement). Try running a 3.25Ω coil on his boat without a resistor and there will be trouble.

Everybody has different input voltage and therefore different ignition system current. We leveled the field with the ballast resistor calculator.

[Administrator]

For an interesting project, try Googling for info on the coils used in today's marinized gasoline engines.

Bill

[hanleyclifford]

Another Spanner in the Works?

Hanley,

The circuit is in series. The current should be the same everywhere in the circuit.

Last night I went out to the boat and installed an ammeter between coil - and the black wire of the EI. At 800 rpms there was a discernible difference of about .1 amp (lower reading at coil - versus coil +). Granted the meters are analog, Chinese and possibly not perfect (though zero lines were correct), but they are both very new. As rpms rose over 1200 I could not discern any difference between their readings. OK, not scientific, could be anecdotal, could be.... Anyway, FWIW